Various forms of holder-gages have been heretofore provided for determining the proper relative positioning and overlap of adjacent portions of siding members. Examples of these various forms of holder-gages are disclosed in U.S. Pat. Nos. 132,159, 367,574, 2,446,950, 2,818,387, 3,490,152 and 3,904,184. However, most of these holder-gages are either not provided for ready support from the last course of siding applied or are not constructed in a manner whereby siding members of the next course may be readily longitudinally, horizontally shifted along the associated building wall into proper position for securement thereto. Accordingly, a need exists for a siding holder-gage which may be readily used by a single person in properly relatively positioning and determining the desired overlap of adjacent courses of siding members.